8,109 research outputs found
A sensitive survey for 13CO, CN, H2CO and SO in the disks of T Tauri and Herbig Ae stars
We use the IRAM 30-m telescope to perform a sensitive search for CN N=2-1 in
42 T Tauri or Herbig Ae systems located mostly in the Taurus-Auriga region.
CO J=2-1 is observed simultaneously to indicate the level of confusion
with the surrounding molecular cloud. The bandpass also contains two
transitions of ortho-HCO, one of SO and the CO J=2-1 line which
provide complementary information on the nature of the emission.
While CO is in general dominated by residual emission from the cloud,
CN exhibits a high disk detection rate % in our sample. We even report CN
detection in stars for which interferometric searches failed to detect
CO, presumably because of obscuration by a foreground, optically thick,
cloud. Comparison between CN and o-HCO or SO line profiles and intensities
divide the sample in two main categories. Sources with SO emission are bright
and have strong HCO emission, leading in general to [HCO/CN].
Furthermore, their line profiles, combined with a priori information on the
objects, suggest that the emission is coming from outflows or envelopes rather
than from a circumstellar disk. On the other hand, most sources have
[HCO/CN], no SO emission, and some of them exhibit clear
double-peaked profiles characteristics of rotating disks. In this second
category, CN is likely tracing the proto-planetary disks. From the line flux
and opacity derived from the hyperfine ratios, we constrain the outer radii of
the disks, which range from 300 to 600 AU. The overall gas disk detection rate
(including all molecular tracers) is , and decreases for fainter
continuum sources.
This study shows that gas disks, like dust disks, are ubiquitous around young
PMS stars in regions of isolated star formation, and that a large fraction of
them have AU.Comment: 31 pages (including 59 figures
Soliton Instabilities and Vortex Streets Formation in a Polariton Quantum Fluid
Exciton-polaritons have been shown to be an optimal system in order to
investigate the properties of bosonic quantum fluids. We report here on the
observation of dark solitons in the wake of engineered circular obstacles and
their decay into streets of quantized vortices. Our experiments provide a
time-resolved access to the polariton phase and density, which allows for a
quantitative study of instabilities of freely evolving polaritons. The decay of
solitons is quantified and identified as an effect of disorder-induced
transverse perturbations in the dissipative polariton gas
Making precise predictions of the Casimir force between metallic plates via a weighted Kramers-Kronig transform
The possibility of making precise predictions for the Casimir force is
essential for the theoretical interpretation of current precision experiments
on the thermal Casimir effect with metallic plates, especially for sub-micron
separations. For this purpose it is necessary to estimate very accurately the
dielectric function of a conductor along the imaginary frequency axis. This
task is complicated in the case of ohmic conductors, because optical data do
not usually extend to sufficiently low frequencies to permit an accurate
evaluation of the standard Kramers-Kronig integral used to compute . By making important improvements in the results of a previous paper by
the author, it is shown that this difficulty can be resolved by considering
suitable weighted dispersions relations, which strongly suppress the
contribution of low frequencies. The weighted dispersion formulae presented in
this paper permit to estimate accurately the dielectric function of ohmic
conductors for imaginary frequencies, on the basis of optical data extending
from the IR to the UV, with no need of uncontrolled data extrapolations towards
zero frequency that are instead necessary with standard Kramers-Kronig
relations. Applications to several sets of data for gold films are presented to
demonstrate viability of the new dispersion formulae.Comment: 18 pages, 15 encapsulated figures. In the revised version important
improvements have been made, which affect the main conclusions of the pape
Dynamical Masses of Low Mass Stars in the Taurus and Ophiuchus Star Forming Regions
We report new dynamical masses for 5 pre-main sequence (PMS) stars in the
L1495 region of the Taurus star-forming region (SFR) and 6 in the L1688 region
of the Ophiuchus SFR. Since these regions have VLBA parallaxes these are
absolute measurements of the stars' masses and are independent of their
effective temperatures and luminosities. Seven of the stars have masses
solar masses, thus providing data in a mass range with little data, and of
these, 6 are measured to precision . We find 8 stars with masses in the
range 0.09 to 1.1 solar mass that agree well with the current generation of PMS
evolutionary models. The ages of the stars we measured in the Taurus SFR are in
the range 1-3 MY, and MY for those in L1688. We also measured the
dynamical masses of 14 stars in the ALMA archival data for Akeson~\&~Jensen's
Cycle 0 project on binaries in the Taurus SFR. We find that the masses of 7 of
the targets are so large that they cannot be reconciled with reported values of
their luminosity and effective temperature. We suggest that these targets are
themselves binaries or triples.Comment: 20 page
Sensitive survey for 13CO, CN, H2CO, and SO in the disks of T Tauri and Herbig Ae stars II: Stars in Oph and upper Scorpius
We attempt to determine the molecular composition of disks around young
low-mass stars in the Oph region and to compare our results with a
similar study performed in the Taurus-Auriga region. We used the IRAM 30 m
telescope to perform a sensitive search for CN N=2-1 in 29 T Tauri stars
located in the Oph and upper Scorpius regions. CO J=2-1 is
observed simultaneously to provide an indication of the level of confusion with
the surrounding molecular cloud. The bandpass also contains two transitions of
ortho-HCO, one of SO, and the CO J=2-1 line, which provides
complementary information on the nature of the emission. Contamination by
molecular cloud in CO and even CO is ubiquitous. The CN detection
rate appears to be lower than for the Taurus region, with only four sources
being detected (three are attributable to disks). HCO emission is found
more frequently, but appears in general to be due to the surrounding cloud. The
weaker emission than in Taurus may suggest that the average disk size in the
Oph region is smaller than in the Taurus cloud. Chemical modeling shows
that the somewhat higher expected disk temperatures in Oph play a direct
role in decreasing the CN abundance. Warmer dust temperatures contribute to
convert CN into less volatile forms. In such a young region, CN is no longer a
simple, sensitive tracer of disks, and observations with other tracers and at
high enough resolution with ALMA are required to probe the gas disk population.Comment: 18 pages, 5 figures, accepted for publication in A&
Exotic and native earthworms in various land use systems of Central, southern and Eastern Uruaguay.
From 1995 to 2001 we conducted several surveys of earthworm populations in 7 land use systems, of varying intensity of disturbance, in Central, Southern and Eastern Uruguay. We evaluated the presence and density of various earthworm genera and species in selected land use systems. Most species found were exotic, belonging to the Lumbricidae (6 species) and Megascolecidae (1 species) families. We found only two native species, one ocnerodrilid and one acanthodrilid. Lumbricids were generally in disturbed agroecosystems, although native species were also found in some disturbed sites. Eukerria sp. had a mean density of 196 ind./m2 in a rice field. The apple orchard, Eucalyptus sp. plantation and natural prairie showed maximum mean densities of 102 ind./m2, 733 ind./m2 and 317 ind./m2, respectively. Our results show the great adaptability of lumbricids to various levels of disturbance and suggest the possibility of a large incidence of exotic worm species in Uruguayan soils
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